Qualify all names in bodies of exported macros with $crate::
Qualify all names in bodies of exported macros with $crate::

The lack of qualification caused odd errors such as:

```
use clap;
let foo = clap::value_t!(matches.value_of("foo"),i u32).unwrap(); # OK
lot bar = clap::value_t!(matches, "bar", u32).unwrap(); # Compile fail
```

but

```
use clap::value_t;
let foo = value_t!(matches.value_of("foo"),i u32).unwrap(); # OK
lot bar = value_t!(matches, "bar", u32).unwrap(); # OK
```

clap-rs/clap,clap-rs/clap,clap-rs/clap